WO 99/35368 discloses a method of radially expanding a string of casing whereby adjacent casings have such axially overlapping portions. In the known method a first casing is lowered into the wellbore and radially expanded by means of an expander mandrel. A second casing is then lowered through the expanded first casing until an upper end part of the second casing is positioned in a lower end part of the first casing. The second casing is subsequently expanded to substantially the same inner diameter as the first casing.
It is a drawback of the known method that the expansion forces required to expand the upper end part of the second casing are very high because, simultaneously with expanding said upper end part, the lower end part of the first casing is to be expanded further. In case the first casing has already been cemented in place, subsequent expansion becomes even more difficult.
In accordance with the invention there is provided a method of radially expanding a tubular element extending into a wellbore, the tubular element having a first section to be expanded to a first diameter and a second section to be expanded to a second diameter, the first diameter being larger than the second diameter, the method comprising:    a) arranging an expander in the wellbore, the expander including a first expander member and a second expander member, wherein the first member has a larger outer diameter than the second member, said members being releasably interconnected;    b) moving the expander through the first tubular section so as to expand the first tubular section to the first diameter;    c) disconnecting the second expander member from the first expander member; and    d) moving the second expander member through the second tubular section so as to be expanded to the second diameter.
It is thereby achieved that the tubular element is expanded to sections of different diameters without having to expand overlapping portions of adjacent tubular elements simultaneously, so that the required expansion forces remain within acceptable limits. It is a further advantage of the method of the invention that the tubular element can be cemented in place before further drilling of the wellbore.
In order to allow the expansion process to be carried out by pulling the expander upwardly through the tubular element, it is preferred that the first tubular section is a lower end part of the tubular element, and the second tubular section is the remaining part of the tubular element.